ReadWriteDiskUsage is incorrect for the zfs driver

[dfr]

It returns a summary for everything visible in the layer where it should return the usage for just this layer, similar to overlay. The zfs clone data set for containers only consumes space for objects which are modified.

We have a good estimate of bytes-used in d.dataset.Used but no value for 'inodes' used (which isn't a terribly important value for zfs).